Coagulation process of PTFE fine powders

ABSTRACT

A continuous coagulation process of PTFE or modified PTFE fine powders, comprising:  
     a1) dilution in a lift of a PTFE latex up to a concentration from 5 to 25% w/w of PTFE, and optional filtration of the obtained diluted latex,  
     b1) latex pressurization in the lift by an inert gas, preferably air, until a relative pressure, related to the atmospheric pressure, in the range 3-40 kg/cm 2  (0.3-4 MPa),  
     c1) addition of an acid electrolyte solution,  
     d1) latex flowing from the mixer through a capillary tube under turbulent flow conditions,  
     e1) granulation (coagulation) of the gel obtained in step d1) by mechanical stirring with a specific power from 1.5 to 10 kW/m 3 , maintaining the stirring until fine powder flotation,  
     f1) separation of the underlying water from the fine powder.

[0001] The present invention relates to a continuous coagulation processof polytetrafluoroethylene (PTFE) prepared by a polymerization processin dispersion or emulsion, to obtain wet fine powders of nonthermoprocessable PTFE (or modified PTFE).

[0002] More specifically, the invention relates to a continuouscoagulation process of PTFE fine powders, comprising theaggregation/gelification of the PTFE latex by a capillary tube, andsubsequent granulation (coagulation) by mechanical stirring.

[0003] Non thermoprocessable fine powders of PTFE or modified PTFEobtained by the process of the present invention are characterized by ahigh apparent density, ≧470 g/l, an average diameter (D₅₀) higher than200 micron, preferably from 400 to 600 micron, and characterized by anarrow distribution of the particle diameters, said distribution,defined as a ratio between the weight of the particles having a diameterin the range 0.7-1.3 times the average diameter of the particles and thetotal weight of the polymer particles, being higher than 50%, preferablyhigher than or equal to 60%.

[0004] It is known in the prior art that PTFE fine powders are obtainedby the polymerization in dispersion. In this process a sufficiently highsurfactant amount is used so as to be able to stabilize the PTFEcolloidal particles and a soft stirring is applied during thepolymerization to avoid the polymer coagulation (precipitation).Subsequently the latex obtained from said process is coagulated and thepowder obtained from the coagulation is called “fine powder”. Theconventional coagulation process comprises the following steps:

[0005] latex dilution with water, optionally addition of a destabilizingelectrolyte,

[0006] latex mechanical stirring to obtain the formation ofaggregates/gels,

[0007] granulation (coagulation) of the aggregates/gels by mechanicalstirring,

[0008] PTFE flotation from water,

[0009] mechanical separation of the wet fine powder from the coagulumwater.

[0010] Said PTFE fine powders are transformed by the lubricatedextrusion process to obtain the manufactured articles.

[0011] The lubricated extrusion process is much more productive andeasily automatizable the more the fine powders are flowing and have ahigh apparent density. Besides, the average particle diameter ispreferably higher than 400 micron and the particle diameter distributionas above defined should be at least higher than 50%, preferably higherthan or equal to 60% to obtain an improved flow.

[0012] Various processes to obtain fine powders, both in batch and in acontinuous way, are known in the prior art. With the batch processesapparent densities of about 400 g/litre (see comparative Examples) orslightly higher are obtained and the particle average diameter (D₅₀) isin the desired range. However the fine powders show the drawback to havea particle diameter distribution as above defined lower than 50%, andtherefore unsuitable. Another drawback of the batch processes is thatfrom the industrial point of view they are more expensive than thecontinuous processes. It is known to the skilled that generally it isnot possible to transform the batch processes to obtain PTFE fine powderin continuous processes.

[0013] In U.S. Pat. No. 3,046,263 a continuous coagulation process ofthe PTFE latexes is described, comprising the following steps:

[0014] phase of strong mechanical stirring of the polymerization latexesusing a specific power of 1-100 CV×sec/gallon (196 kJ/m³-19,600 kJ/m³) ,preferably using a centrifugal pump with an average time of the latexresidence in the pump of 2 seconds;

[0015] aggregate or gel formation by passing through a capillary tubehaving hydraulic resistance of 0.5-20 p.s.i. (˜3.4-136 kPa);

[0016] granulation in the presence of air by mechanical stirring withspecific power of 0.25-50 CV×sec/gallon (49 kJ/m³-9,800 kJ/m³);

[0017] subsequent separation of the PTFE fine powder from water.

[0018] The drawback of said process is that the capillary tube is easilyobstructed whereby the process must be often interrupted to clean thecapillary tube. Furthermore it has been found by the Applicant that alsoin the first phase of the latex preparation which must be introduced inthe capillary tube for gelification, the partial formation of PTFEgranules takes place causing the obstruction of the centrifugal pump.This is a further drawback of the above process.

[0019] Another continuous process which avoids the drawbacks of theprocess described in the previous patent is reported in U.S. Pat. No.5,977,295. In said process a capillary tube is not used, sinceconsidered unsuitable for a continuous process due to the abovedrawbacks. The PTFE latex deriving from the polymerization in emulsionis fed to a high shear apparatus having rotating elements to obtainaggregates/gels (gel phase). The so obtained gel phase is fed to thecoagulation phase in a column by applying shear forces to the gel phase.According to U.S. Pat. No. 5,977,295 by this process high apparentdensities, a good average particle diameter are obtained, but theparticle diameter distribution is not mentioned. The drawback of saidprocess resides in that a very complicated and expensive high shearapparatus is used, which requires rotor peripherical rates of the orderof 2 to 30 m/s and free space between stator and rotor of about 0.5 mm.This requires the water fluxing on the rotating parts for the correctworking of the shear apparatus and of the process described therein.Besides, even though a so complicated apparatus is used, the shear isnot perfectly uniform whereby it is not possible to obtain a narrowdistribution of the particle diameters. This leads to the abovedrawbacks.

[0020] The need was felt to have available a continuous process for thepreparation of wet PTFE fine powders, overcoming the drawbacks of theprior art and not requiring the use of complicated and expensiveapparatus to obtain high shears to reduce the unhomogeneity of theobtained fine powders, and allowing to obtain PTFE fine powders havingan improved flow and high apparent density and narrow distribution ofthe particle diameters, as above defined, higher than 50%, preferablyhigher than or equal to 60%.

[0021] An object of the present invention is a continuous coagulationprocess of PTFE or modified PTFE fine powders, comprising:

[0022] a1) dilution in a lift of a PTFE or modified PTFE latex obtainedfrom the polymerization in dispersion (emulsion) up to a concentrationfrom 5 to 25% w/w of PTFE, preferably from 8 to 20% w/w of PTFE ormodified PTFE; and

[0023] optional filtration of the obtained diluted latex,

[0024] b1) latex pressurization in the lift by an inert gas, preferablyair, until a relative pressure, referred to the atmospheric pressure, inthe range 3-40 kg/cm² (0.3-4 MPa), preferably from 5 to 20 kg/cm² (0.5-2MPa), and still more preferably from 7 to 15 kg/cm² (0.7-1.5 MPa),

[0025] c1) addition of an acid electrolyte solution, preferably nitricacid, to the PTFE or modified PTFE latex, in a in-line mixer, so thatthe pH is from 1 to 4, preferably from 1.5 to 3; the latex concentrationand the pH being such as not to cause the formation of aggregates/gelsin the in-line mixer,

[0026] d1) latex flowing from the mixer through a capillary tube underturbulent flow conditions, having a Reynolds number higher than 3,000,preferably higher than 5,000,

[0027] e1) granulation (coagulation) of the gel obtained in step d1) bymechanical stirring with a specific power from 1.5 to 10 kW/m³,maintaining the stirring until fine powder flotation,

[0028] f1) separation of the underlying water from the wet fine powder.

[0029] In the step d1) to obtain turbulent flow conditions of the latex,one preferably works as follows:

[0030] the capillary tube has an internal diameter such that the totalcapillary tube hydraulic resistance under the process conditions causesa pressure fall between the capillary tube ends from 3 to 40 kg/cm²(0.3-4 MPa), preferably from 5 to 20 kg/cm² (0.5-2 MPa), and still morepreferably from 7 to 15 kg/cm² (0.7-1.5 MPa),

[0031] the capillary tube length is from 0.1 to 30 m, preferably from0.3 to 15 m, and still more preferably from 1 to 10 m,

[0032] the latex/gels rate inside the capillary tube is in the range2-15 m/sec,

[0033] the capillary tube diameter is generally from 2 to 20 mm,preferably from 3 to 10 mm.

[0034] In a preferred embodiment the capillary tube length ranges from0.1 to 3 m, still better from 0.2 to 1 m.

[0035] The combination of the capillary tube diameter, the latexconcentration, the electrolyte pH and the latex linear rate through thecapillary tube, must be such as to guarantee a turbulent flow of thelatex inside the capillary tube.

[0036] In step d1) aggregation/gelification takes place. The capillarytube length as above defined is such to guarantee a completeaggregation/gelification of the latex and the absence of formation ofthe fine powder granules inside the capillary tube.

[0037] In FIG. 1 it is reported a preferred embodiment of the apparatuscomprising the capillary tube for obtaining, starting from the latex, apolymer gel. The numbers have the following meaning:

[0038]1 Inlet for the latex.

[0039]2 Mixing chamber.

[0040]3 Pipe for electrolyte feeding.

[0041]4 Convergent section of 2.

[0042]5 Capillary tube.

[0043]6 Divergent section.

[0044]7 Outlet for the gel.

[0045] The latex entering from 1 is mixed with the electrolyte comingfrom 3 in the mixing chamber 2. Then the latex mixed with theelectrolyte in chamber 2, enters in section 4 connecting the mixingchamber 2 with the capillary tube 5. The angle of the profile of theconvergent section 4 can be from about 5° to 15°, preferably of 10°.Then the latex mixture enters in the capillary tube 5, wherein gelformation takes place. The gel so formed passes to section 6. The angleof the profile of this section is not particularly limited, and ingeneral ranges from 10° to 20°. The gel at the end of 6, through theoutlet 7, goes into the granulation apparatus (step e1) of the processof the invention).

[0046] By aggregates/gels it is meant that the polymer particles aredipped in the liquid phase and linked by crossed bonds so to form athick network. The aggregates/gels properties significantly depend onthe interactions of said two components (polymer and liquid). The liquidprevents the polymer network from being transformed into a compact massand the polymer network prevents the liquid from coming out from theaggregates/gels. Depending on the chemical composition and other factorsthe consistence of the aggregates/gels varies from fluids to ratherstiff solids.

[0047] At the end of step b1), while the previously pressurized latex isdischarged in the mixer of step c1), a second lift is used which is fedwith latex to be diluted according to step a1). When the first lift hasbeen emptied, the latex fed into the second lift is at the end of stepb1), and therefore the latex is fed again in the first lift.

[0048] For step e1) one works for example with the columns described inthe prior art to have a continuous process, for example vessels having aheigth/diameter ratio higher than 2, equipped with stirrer (see theprocesses described in the above patents).

[0049] The PTFE or modified PTFE fine powders obtained from the processof the present invention are particularly suitable to the transformationby lubricated extrusion to obtain manufactured articles.

[0050] By PTFE according to the present invention the TFE homopolymer ismeant, by modified PTFE, TFE copolymers with one or more comonomers aremeant, having at least one unsaturation of ethylene type in an amountfrom 0 to 3% by moles, preferably from 0.01 to 1% by moles.

[0051] The comonomers which can be used are of both hydrogenated andfluorinated type.

[0052] Among the hydrogenated comonomers, ethylene, propylene, acrylicmonomers, for example methylmethacrylate, (meht)acrylic acid,butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such forexample styrene can be mentioned.

[0053] Among the fluorinated comonomers it can be mentioned:

[0054] C₃-C₈ perfluoroolefins, such as hexafluoropropene (HFP);

[0055] C₂-C₈ hydrogenated fluoroolefins, such as vinyl fluoride (VF),vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene,perfluoroalkylethylene CH₂═CH—R_(f), wherein R_(f) is a C₁-C₆perfluoroalkyl;

[0056] C₂-C₈ chloro- and/or bromo- and/or iodo-fluoroolefins, such aschlorotrifluoroethylene (CTFE);

[0057] (per)fluoroalkylvinylethers (PAVE) CF₂═CFOR_(f), wherein R_(f) isa C₁-C₆ (per)fluoroalkyl, for example CF₃, C₂F5, C₃F₇;

[0058] (per)fluorooxyalkylvinylethers CF₂═CFOX, wherein X is: a C₁-C₁₂alkyl, or a C₁-C₁₂ oxyalkyl, or a C₁-C₁₂ (per)fluorooxyalkyl having oneor more ether groups, for example, perfluoro-2-propoxy-propyl;

[0059] fluorodioxoles, preferably perfluorodioxoles;

[0060] non conjugated dienes of the type:

[0061] CF₂═CFOCF₂CF₂CF═CF₂,

[0062] CFX¹═CX²OCX³X⁴OCX²═CX¹F

[0063]  wherein X¹ and X², equal to or different from each other, are F,Cl or H; X³ and X⁴, equal to or different from each other, are F or CF₃,which during the polymerization cyclopolymerize;

[0064] fluorovinylethers (MOVE) of general formula:

[0065] CFX_(AI)═CX_(AI)OCF₂OR_(AI) (A-I) wherein R_(AI) is a C₂-C₆linear, branched or C₅-C₆ cyclic (per)fluoroalkyl group, or a C₂-C₆linear, branched (per) fluoro oxyalkyl group, containing from one tothree oxygen atoms; when R_(AI) is a fluoroalkyl or a fluorooxyalkylgroup as above it can contain from 1 to 2 atoms, equal or different,selected from the following: H, Cl, Br, I; X_(AI)═F, H; the compounds ofgeneral formula: CFX_(AI)═CX_(AI)OCF₂OCF₂CF₂Y_(AI) (A-II) , whereinY_(AI)═F, OCF₃; X_(AI) as above are preferred; in particular (MOVE I)CF₂═CFOCF₂OCF₂CF₃ (A-III) and (MOVE II) CF₂═CFOC—F₂OCF₂CF₂OCF₃ (A-IV)are preferred.

[0066] The continuous process of the present invention, as said, allowsto obtain PTFE fine powders having a narrow distribution (as above) ofthe particle diameter, whereby the fine powders show an improved flow.This property is combined with a high apparent density and with anaverage diameter preferably higher than 400 micron.

[0067] The continuous process of the invention leads to fine powderswith reproducible properties. In the capillary tube used in the processaccording to the present invention, obstructions do not occur, evenafter long working periods. Besides from the industrial point of viewthe process does not require very expensive apparatuses having a highshear as above indicated.

[0068] The following Examples illustrate the invention but withoutlimiting the scope thereof.

EXAMPLES Characteriization Methods

[0069] Distribution of the particle diameter (%)

[0070] The particle diameter distribution is calculated determining theratios by weight between the particles having a diameter in the rangefrom 0.7 to 1.3 times the average particle diameter on the total of theparticles, and multiplying by 100. The particle diameter is determinedby laser scattering using as device the Coulter® LS 230 instrument. Theinstrument directly gives the distribution curve. From the distributioncurve it is possible to directly calculate the weight of thedistribution particles having the diameters in the above consideredrange.

[0071] Determination of the apparent density

[0072] The ASTM D 4895-89 method is used.

[0073] Determination of the average diameters (D₅₀)

[0074] The PTFE fine powder is dispersed in water with non ionicsurfactant Triton®X100 and the particle diameter is determined by laserscattering using as device the Coulter®LS 230 instrument. The instrumentdirectly gives the distribution curve from which the instrumentcalculates the D₅₀ and the distribution as above reported.

[0075] Determination of the dispersion density

[0076] The dispersion density is calculated as average density from thefollowing formula:$\frac{100}{{\Sigma \quad {x/d}\quad e\quad n\quad s\quad i\quad t\quad y_{w\quad a\quad t\quad e\quad r}} + {{y/d}\quad e\quad n\quad s\quad i\quad t\quad y_{P\quad T\quad F\quad E}}}$

[0077] wherein:

[0078] x=percentage by weight of water in the dispersion,

[0079] y=percentage by weight of PTFE in the dispersion.

[0080] Viscosity calculation

[0081] The dispersion viscosity is determined by the following formula(Einstein formula):

viscosity _(medium)×(1+2.5Φ)

[0082] wherein the viscosity_(medium) is the water viscosity and Φ=PTFEvolumetric concentration in the dispersion.

Example A

[0083] Preparation of the PTFE latex

[0084] 600 parts by weight of degassed water, 1.33 parts of an aqueoussolution of ammonium perfluorooctanoate at 30% w/w in ammoniumperfluorooctanoate, 3 parts of an aqueous solution of ammoniumpersulphate at 0.2 % w/w in ammonium persulphate are fed into a reactorequipped with mechanical stirrer, previously put under vacuum. Thereactor is pressurized with TFE up to a pressure of 20 bar (2 MpA) at atemperature of 30° C. Then 3 parts of an aqueous solution of(NH₄)₂Fe(SO₄)₂×6H₂O (Mohr salt) at a concentration of 0.3% by w. arefed.

[0085] When the pressure in the reactor has decreased of 0.5 bar (5×10⁴Pa) one starts to feed TFE so as to maintain a constant pressure of 20bar inside the reactor. In the meantime the reactor internal temperatureis increased up to 85° C. at a rate equal to 1° C./min. During thereaction 3.5 parts of the above ammonium perfluorooctanoate aqueoussolution (surfactant) are fed into the reactor.

[0086] After 50 minute from the start, the TFE feeding is stopped, thereactor vented, cooled and lastly discharged. The discharged latex has aconcentration of 510 g of PTFE/litre of water.

Example 1

[0087] Process according to the present invention by diluting in stepa1) the latex to a concentration of 120 g/l, and using in step b1) apressure of 15 bar (1.5 MPa), in step d1) a capillary tube length(diameter 3 mm) of 7 meters

[0088] The latex obtained as in Example A is diluted in a lift (lift 1)having a 500 litre capacity until obtaining a concentration of 120 g/l(10.7% in solid), the latex is diluted so that the temperature ismaintained at 22° C.

[0089] After the dilution the lift 1 is pressurized with compressed airat a pressure of 1.5 Mpa related to the atmospheric pressure.

[0090] The latex is mixed, in a mixer in-line, with a HNO3 solution at3% by weight and a latex having a concentration of 110 g/l with pH 2 isobtained, the temperature is 22° C.

[0091] No aggregation/gelification of the latex during this last stephas been observed.

[0092] The latex is let flow through a capillary tube having a diameterof 3 mm, length of 7 meters with a 6.4 m/sec rate. By operating in saidway the complete aggregation/gelification of the latex is guaranteedwith absence of granules of coagulated polymer (fine powder).

[0093] The obtained gel is granulated under mechanical stirring having aspecific power of 2.7 KW/m³, until complete granulation and flotation ofthe fine powder. The wet fine powder is separated from water and dried.

[0094] The obtained fine powder has the properties of apparent density,average diameter, and diameter distribution as shown in Table 1.

[0095] When the content of lift 1 is ended, the lift 2 is inserted,having the same capacity of lift 1, to have a continuous process.

Example 2

[0096] Process according to the present invention by diluting in stepa1) the latex to a concentration of 120 g/l, and using in step b1) apressure of 10 bar (1 MPa), in step d1) a capillary tube length(diameter 3 mm) of 7 meters

[0097] One operates as in Example 1, but with the following changes.

[0098] The latex obtained in Example A is diluted in a lift having a 500litre capacity until obtaining a concentration of 120 g/l (10 7% insolid), the latex is diluted at the temperature of 22° C.

[0099] After the latex dilution, the lift is pressurized with compressedair at 1 Mpa related to the atmospheric pressure.

[0100] The latex is mixed, in a mixer in-line, with a HNO3 solution at3% obtaining a latex having a concentration of 110 g/l with pH 2, thetemperature is 22° C. Under said conditions there is no latexgelification.

[0101] The latex is then let flow in a capillary tube having a diameterof 3 mm, length of 7 metres with a 5 m/sec rate. By operating in saidway the complete gelification of the latex is guaranteed with absence ofgranules of coagulated polymer (fine powder).

[0102] The obtained gel is granulated under mechanical stirring with aspecific power of 2.5 KW/m³, until complete granulation and flotation ofthe fine powder. The wet fine powder is separated from water and dried.

[0103] The obtained fine powder has the properties of apparent density,average diameter, and diameter distribution as shown in Table 1.

Example 3

[0104] Process according to the present invention by diluting in stepa1) the latex to a concentration of 80 g/l, and using in step b1) apressure of 15 bar (1.5 MPa), in step d1) a capillary tube length(diameter 3 mm) of 19 meters

[0105] One operates as in Example 1, but with the following changes.

[0106] The latex obtained as in Example A is diluted in a lift having a500 litre capacity to a concentration of 80 g/l (7.4% in solid), thedilution is carried out at the constant temperature of 22° C.

[0107] After the dilution, the lift is pressurized with compressed airat a pressure of 1.5 Mpa related to the atmospheric pressure.

[0108] The latex is mixed, in a mixer in-line, with a HNO3 solution at3% by weight so that a latex having a concentration of 75 g/l with pH2.5 is obtained, at the temperature of 22° C. Under said conditionsthere is no latex gelification.

[0109] The latex is then let flow through a capillary tube having adiameter of 3 mm, length of 19 meters with a 4 m/sec rate. By operatingin said way the complete gelification of the latex is guaranteed withabsence of granules of coagulated polymer (fine powder).

[0110] The obtained gel is granulated under mechanical stirring,specific power of 2.2 KW/m³, until complete granulation and flotation ofthe fine powder. The wet fine powder is separated from water and dried.

[0111] The obtained fine powder has the properties of apparent density,average diameter, and diameter distribution as shown in Table 1.

EXAMPLE 1c

[0112] (comparative)

[0113] Batch process to obtain the fine powder from a PTFE latexaccording to the prior art without capillary tube

[0114] In a 50 litre reactor 15 litres of latex of Example A and waterare introduced, until obtaining 22 litres of latex having concentrationof 160 g/l (13.7% in solid). The latex is diluted so that after dilutionthe temperature is 22° C.

[0115] The mixture is added under stirring with a HNO₃ solution at 20%by weight so to bring the latex pH to 2.5.

[0116] The mechanical stirring causes the aggregation of the colloidalparticles. First there is gelification, then granulation and lastlyflotation of the coagulated powder. The obtained fine powder isseparated from water and dried.

[0117] The obtained fine powder has the properties of apparent density,average diameter and diameter distribution as shown in Table 1.

[0118] A powder having an apparent density lower than 470 g/l and adistribution of the particle diameters lower than 50% is obtained.

EXAMPLE 2c

[0119] (comparative)

[0120] Continuous process according to U.S. Pat. No. 3,046,263

[0121] One operates as in Example 1, but with the following changes.

[0122] The latex obtained in Example A is diluted until obtaining aconcentration of 120 g/l (10.7% in solid). The latex is diluted so thatthe final temperature after dilution is 22° C. The mixture is addedunder stirring with a HNO₃ solution at 20% by weight so to bring thelatex pH to 2.5.

[0123] The latex is fed to the capillary tube (diameter 3 mm, length 7m), by using a centrifugal pump. By operating in this way the pump andthe capillary tube become obstructed and it is not possible to proceedfurther. TABLE 1 Concentration of the diluted latex, relative pressurein the lift, pH after dilution in the mixer with a HNO₃ solution, pH andcapillary tube length and corresponding properties of the obtained finepowders: apparent density, average diameter (D₅₀) and % by weight in thepowders of the particles having a diameter from 0.7 to 1.3 times theaverage diameter of the powder particles Latex Capillary conc. Relativetube Apparent Particle after dil. pressure length density distributionEx. g/litre (bar) pH (m) (kg/m³ D₅₀ (μm) % by weight 1 120 15 2 7 510420 65 2 120 10 2 7 480 470 60 3 80 15 2.5 19  520 500 60 1c comp 160 —2.5 — 410 470 45 2c comp 120 — 2.5 7 — — —

1. A continuous coagulation process of PTFE or modified PTFE finepowders, comprising: a1) dilution in a lift of a PTFE latex obtainedfrom the polymerization in dispersion (emulsion) to a concentration from5 to 25% w/w of PTFE, preferably from 8 to 20% w/w of PTFE or modifiedPTFE; and optional filtration of the obtained diluted latex, b1) latexpressurization in the lift by an inert gas, preferably air, up to arelative pressure, referred to the atmospheric pressure, in the range3-40 kg/cm² (0.3-4 MPa) , preferably 5-20 kg/cm² (0.5-2 MPa) , and stillmore preferably 7-15 kg/cm² (0.7-1.5 MPa), c1) addition of an acidelectrolyte solution, preferably nitric acid, to the PTFE latex, in ain-line mixer, so that the pH is from 1 to 4, preferably from 1.5 to 3,d1) latex flowing from the mixer through a capillary tube underturbulent flow conditions, having a Reynolds number higher than 3,000,preferably higher than 5,000, e1) granulation (coagulation) of the gelobtained in step d1) by mechanical stirring with a specific power from1.5 to 10 kW/m³, maintaining the stirring until fine powder flotation,f1) separation of the underlying water from the fine powder.
 2. Aprocess according to claim 1, wherein to obtain turbulent flowconditions of the latex, in the step d1) one works as follows: the totalcapillary tube hydraulic resistance under the process conditions causesa pressure fall between the capillary tube ends from 3 to 40 kg/cm²(0.3-4 MPa) , preferably from 5 to 20 kg/cm² (0.5-2 MPa) , and stillmore preferably from 7 to 15 kg/cm² (0.7-1.5 MPa), the capillary tubelength is from 0.1 to 30 m, preferably from 0.3 to 15 m, and still morepreferably from 1 to 10 m, the latex/gels rate inside the capillary tubeis in the range 2-15 m/sec, the capillary tube diameter being from 2 to20 mm, preferably from 3 to 10 mm.
 3. A process according to claims 1-2,wherein the TFE copolymers contain one or more comonomers, ofhydrogenated and/or fluorinated type, having at least one unsaturationof ethylene type in an amount from 0 to 3% by moles, preferably from0.01 to 1% by moles.
 4. A process according to claim 3, wherein thehydrogenated comonomers are selected from ethylene, propylene, acrylicmonomers, preferably methylmethacrylate, (meht)acrylic acid,butylacrylate, hydroxyethylhexylacrylate, styrene monomers.
 5. A processaccording to claim 3, wherein the fluorinated comonomers are selectedfrom the following: C₃-C₈ perfluoroolefins, preferably hexafluoropropene(HFP); C₂-C₈ hydrogenated fluoroolefins, selected from vinyl fluoride(VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobuteneand perfluoroalkylethylene CH₂═CH—R_(f), wherein R_(f) is a C₁-C₆perfluoroalkyl; C₂-C₈ chloro- and/or bromo- and/or iodo-fluoroolefins,preferably chlorotrifluoroethylene (CTFE); (per)fluoroalkylvinylethers(PAVE) CF₂═CFOR_(f), wherein R_(f) is a C₁-C₆ (per)fluoroalkyl,preferably CF₃, C₂F5, C₃F₇; (per) fluoro-oxyalkylvinylethers CF₂═CFOX,wherein X is: a C₁-C₁₂ alkyl, a C₁-C₁₂ oxyalkyl, a C₁-C₁₂(per)-fluorooxyalkyl having one or more ether groups, preferablyperfluoro-2-propoxy-propyl; fluorodioxoles, preferablyperfluorodioxoles; non conjugated dienes of the type:CF₂═CFOCF₂CF₂CF═CF₂, CFX¹═CX²OCX³X⁴OCX²═CX¹F wherein X¹ and X², equal toor different from each other, are F, Cl or H; X³ and X⁴, equal to ordifferent from each other, are F or CF₃, which during the polymerizationcyclopolymerize; fluorovinylethers (MOVE) of general formula:CFX_(AI)═CX_(AI)OCF₂OR_(AI) (A-I) wherein R_(AI) is a C₂-C₆ linear,branched or C₅-C₆ cyclic (per)fluoroalkyl group, or a C₂-C₆ linear,branched (per)fluoro oxyalkyl group, containing from one to three oxygenatoms; when R_(AI) is a fluoroalkyl or a fluorooxyalkyl group as aboveit can contain from 1 to 2 atoms, equal or different, selected from thefollowing: H, Cl, Br, I; X_(AI)═F, H; the compounds of general formula:CFX_(AI)═CX_(AI)OCF₂OCF₂CF₂Y_(AI) (A-II) , wherein Y_(AI)═F, OCF₃;X_(AI) as above are preferred; in particular (MOVE I) CF₂═CFOCF₂OCF₂CF₃(A-III) and (MOVE II) CF₂═CFOC—F₂OCF₂CF₂OCF₃ (A-IV) are preferred. 6.Non thermoprocessable fine powders of PTFE or modified PTFE obtainableby the process of claims 1-5, having: apparent density, ≧470 g/l,average diameter (D₅₀) higher than 200 micron, preferaably from 400 to600 micron, distribution of the particle diameters, defined as ratiobetween the weight of the particles having a diameter from 0.7 to 1.3times with respect to the average particle diameter and the total weightof the particles, higher than 50%, preferably higher than or equal to60%.